Dyson sphere look alikes updated Dec. 17, 2010 D. Carrigan carrigan@fnal.gov (subject line must be sensible)

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Introduction

A number of astronomical objects have infrared signatures somewhat like a Dyson sphere. These include stars with thick dust shells, regions of dust, very young stars that form in regions of dust, Miras stars, planetary nebula, asymptotic giant branch stars (AGB), and post AGB stars. (Click on Look alikes for table below of some of these.) Dyson sphere search has to rule out these more conventional and plausible classifications to identify interesting Dyson sphere candidates. The Calgary Group has complied a comprehensive atlas of all of the available IRAS Low Resolution Spectra. This is discussed in "Classification and Identification of IRAS Sources with Low-Resolution Spectra", Kwok, Volk, and Bidelman, ApJS, 112, 557 (1997).

They break LRS sources into a number of categories including the ones below (U, F, C, and H) that might identify Dyson sphere candidates. While comprehensive, this is not necessarily the last word on IR spectra and identification since more data is now available from facilities such as the Spitzer Space Telescope and 2MASS. The table gives some of the characteristics of the classifications that might contain Dyson spheres.

Calgary Classification

Description

Astronomical objects

U

"Unusual spectra showing a flat continuum"

Type of object generally unkown according to Calgary Atlas of LRS sources

F

Featureless

O or C rich stars with small amounts of dust

C

Carbon stars

Late type stars such as evolved cool giants with circumstellar shells or clouds of carbon dust material. Typically these have small optical depths so a percent or so of the emission is in the infrared.

H

Red continuum with absorption

H II regions, planetary nebula, reflection nebula

IRAS 06176-1036 "Red Rectangle">>>

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Miras	Named after the famous long period variable Mira. Miras are old stars that have evolved into so-called asymptotic giant branch stars. Miras are cooler and much larger than the sun. Typically at a late stage of the stars evolution the outer atmosphere can be blown away so that this phase of the stellar life is relatively short (tens of thousands of years). Molecules such as SiO form as the gas condenses. Typically the infrared signature is the sum of many Planck spectra. In many cases the dust cloud is not thick enough to hide the star. This combination of a visible star and sum of many Planck distributions rules out the possibility that such a star is a pure Dyson sphere. For one explanation of a Miras star see the Harvard-Smithsonian Miras site.
OH/IR Maser	The gas cloud from a Miras star can give rise to hydoxyl ions (OH) and SiO (silicon monoxide). The dust formed from silicon monoxide heats up and excites the hydroxyl gving rise to an inverted population of elevated molecular states. This inverted population can form a natural maser resulting in very strong emission at radio frequencies. An OH/IR signal is evidence against a possible Dyson sphere. For more information on astronomical masers see the Hartebeesthoek radio telescope site.
Protostars and young stars	Typically star formation occurs in regions where there is lots of dust such as Orion. A typical distributions would be the combination of many Planck spectra. This is not the signature of a pure Dyson sphere.
Brown dwarfs	Brown dwarfs are failed stars with masses less than 0.075 the mass of the sun. This mass is too small to ignite hydrogen fusion. The heat flux of the brown dwarf is due to the gravitational assembly energy and dissipates over several hundred million years. Typical temperatures are in the 1000 degree Kelvin range, too high for normal life. This temperature is higher than most anticipate for a Dyson sphere and the luminosity is lower.
Galaxies	Characteristically galaxies detected by the IRAS satellite are relatively nearby and are not point sources. Even a nearby Dyson sphere should be a point source in any infrared telescope.